I suspect that you are wrong, and that most incest happens between same-age adolescent or young adult siblings (and even more between cousins if you count cousins). But I don't have any actual data to back that up either, and would be amazed if any worthwhile data even exist.
34% of reported sexual assault on children take place by family members.
Even "a lot" could reasonably be treated as having been a statement about the proportion, but it would not have been a precise enough statement to argue about. And "a lot" could also be treated as talking about an absolute number, in which case the question of whether something was "a lot" would be a total matter of personal opinion.
"Most", on the other hand, has a very clear meaning in terms of relative numbers... which that RAINN link does nothing to support.
"Arguing in good faith" includes not trying to quietly rewrite your position in the middle.
AND the original post explicitly limited the field to adults. So even that had already been dealt with.
You are operating in the proud rhetorical tradition that calls all LGBT people "groomers"... and I'm done with you.
And why, pray, should they come out only to face unfounded accusations of rape and child abuse?
You missed that it's a totally different statistic. Even if it was 100% of reported sexual assault on children is by family members, it could still be that incest between consenting adults is more common.